Capturing images of features of a substrate such as scratches formed into an upper surface of a semiconductor wafer can be a very difficult task. The illumination techniques used to obtain brightfield 2D images of the surface of the wafer for inspection purposes very often do not provide sufficient contrast with respect to the scratches that are of interest. Accordingly, it can be difficult to use a standard brightfield 2D inspection system to inspect a substrate for fine scratches and marks. Many practitioners resort to other types of illumination such as darkfield illumination. Darkfield illumination is well suited to picking out small discontinuities such as fine scratches and marks in the substantially specular surface of a semiconductor wafer.
Providing darkfield illumination in a 2D inspection system is well-known. Even so, additional illumination arrangements add cost and complexity to a 2D inspection system. Accordingly, imaging small features such as scratches and other defects formed into the surface of a substrate such as a semiconductor wafer using pre-existing brightfield illumination is desirable.